Our marriage is collapsing

The last mc was December 15/2015 and since then I've been working on getting healthier and have lost 35lbs, quit smoking and started therapy for some long term issues.

Well a few months ago he suggested that we start trying again. The thing is we've only had sex once since December and it wasn't even sex. He went soft as soon as he tried to put it in. I've tried talking to him but he says he's tired and then I find him watching porn and masturbating after I've gone to bed. I've chalked it up to he's not attracted to me anymore or he's not satisfied anymore. I've initiated every time and yet he instantly goes soft or falls asleep. I'm ready to give up and walk away. Not just about the sex but the breakdown of the relationship. All we do is fight. It's not healthy for our son and it's not healthy for us. 

Thanks for listening